Time delay blade-type and ferrule-type fuses have heretofore commonly comprised a cylindrical housing of insulating material having terminals, blades or caps, which extend generally axially from the opposite outer ends of a cylindrical housing. Connected between these terminal blades or caps within the housing are a series of axially spaced interconnected elements including a pair of current-heatable, short circuit protection strips respectively located in arc-quenching sand- or powder-filled outermost compartments, and heat and current conducting elements and spring elements in a central compartment which does not have any sand or powder therein. The short circuit protection strips will melt under short circuit conditions, and the arc-quenching sand or powder quickly quenches the resulting arc.
The elements in the central compartment usually include a thermal mass which accumulates heat generated in the current heatable strips in the outermost compartments. In some cases, the thermal mass forms both a heat conducting and a heat accumulating element. One of the current-conducting elements in the central compartment is sometimes an axially movable plunger element guided for movement along a guide element, and secured through heat-meltable solder junctions to both the guide element and a portion of one of the current-heatable short circuit protection strips in one of the end compartments. In one prior art design, the plunger projected into one of the sand- or powder-filled end compartments and was secured therein to the latter strip through one of the heat meltable junctions referred to.
In prior art initially compressed or extended coil springs are generally provided in the central compartment to place the plunger, connected through the heat meltable junctions to other elements, under spring compression or tension. Thus, when the thermal mass accumulates sufficient heat under a modest overload (e.g. 135% overload) to melt these solder junctions, the spring force pulls or pushes a plunger to quickly separate the current-carrying portions of the fuse. In the prior art fuses where an extension of the plunger extends into one of the end compartments, the arc formed by the separation of this extension from the heat-meltable junction results in an arc which desirably is extinguished by the arc-quenching sand or powder in that compartment. This feature is desirably included in the preferred form of the invention.
In many of these prior art fuse designs, because of the design and relationship of the various elements in the central compartment thereof, automation of the fuse assembly was difficult to achieve. Also, the outermost and central compartments of these fuses are often ill-defined by disk-shaped, insulating washers fitted into the housing. Many of these washers were not stably held in position in prior art fuses, so that they sometimes shifted in position, leaving clearance spaces for the sand or powder in the outermost compartments to leak into the central compartment thereof.
In the most preferred form of the invention, disk-shaped insulating washers are provided which are stably supported, and one of them performs a unique function of holding a depressible coil spring in place in the central compartment. Because of this and other features of the invention, there is provided a fuse of very simple and reliable construction, characterized by simplicity of assembly. Also, because of the construction of the basic fuse unit, a plurality of parallel connected fuse units to achieve a high current rating can be readily assembled within a common housing.
In connection with the features of the invention where one of the compartment-defining washers has an added function of retaining a coil spring in a depressed condition (although a broader aspect of the invention does not require the washer to perform this compartment-defining function), the disk-shaped washer is preferably provided with a central opening having a circular central section from which there project, from diametrically opposite sides thereof, aligned slots which correspond in shape and size to the cross section of the adjacent current-heatable short circuit protection strip.
The plunger or other axially movable current conducting element to be located in the central compartment has an axially facing shoulder against which one end of the coil spring to be depressed is placed before the washer is placed on the initial subassembly of the various current conducting and thermal mass elements held together by various solder junctions described. The washer described is then oriented to align the washer opening slots with the short circuit protection strip adjacent the other end of the spring. The washer is then inserted over and slid axially inwardly along the strip first to a point where the washer compresses the coil spring, and then further is passed beyond the strip to pressingly engage another axially facing washer support shoulder on the plunger, at which point the circular central section of the washer opening surrounds a circular extension of the plunger. The coil spring being thus compressed to the desired degree, the washer is then rotated to bring the radially extending slots referred to out of alignment with the current-heatable strip so that the washer then is closely sandwiched between the washer support shoulder of the plunger and the inner end of the strip, and is further securely held in place by the outward force exerted by the compressed coil spring. The washer support shoulder is configured sufficiently wide to completely facingly obstruct the washer slots, thereby preventing leakage of arc-suppressing filler.
Where a second current-heatable strip is to be employed connected to the other end of the plunger assembly and occupying a similarly sealed compartment at the opposite end of the structure a second partition wall is preferably formed by a similar washer with an opening having a circular central section and aligned slots as above described and extending therefrom so that it can be similarly slid axially along the other current-heatable short circuit protection strip. When the washer passes beyond the inner end of the latter strip, it surrounds a circular extension of a member to be located within the central compartment of the fuse housing. Rotation of the washer will then bring the washer in a preferably secure position held closely captive between an axially facing shoulder of the latter member and the inner end of the latter strip. The sandwiching shoulders and strip ends described brace and rigidify the washers involved so that they will remain in position and serve as reliable sand or filler retaining partition walls when the assembly is fitted into the fuse housing.
In accordance with a specific aspect of the invention, the latter member forms a cylindrical plunger guide member which fits within and is located at the open end of a guide member-receiving bore of the plunger described. The plunger guide member is secured to the plunger thereat through a solder junction which melts about the same time as the solder junction which connects the other end of the plunger with the current heatable short circuit protection strip at the opposite end of the fuse.
In accordance with a form of the invention where the fuse includes two or more fuse unit subassemblies connected in parallel and mounted within a common housing, a similar multi-fuse unit subassembly is formed through the aid of a pair of fixedly held axially spaced washers similar in some respects to those described but having a number of openings, each having a central circular section from which project one or more pairs of aligned strip-receiving slots conforming to the cross section of one or more strips which are to be received thereby. In this form of the invention, the individual fuse unit subassemblies are mounted within the axially spaced washers by insertion of each such subassembly with uncompressed coil spring in place thereon into the corresponding aligned washer openings and then rotating the same into position where the springs are held in a compressed condition to complete an overall assembly containing two or more fuse unit subassemblies to be connected in parallel when the blade or other terminals are secured to the outer ends of the strips involved. Another aspect of the invention is the novel assembly method for both forms of the fuse described.
The above and other features of the invention will become apparent upon making reference to the specification to follow the claims and the drawings.